<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

</body>

</html>
<script>
    let str = 'Hello World'
    let letter = (str) => {
        let res = [...str].map((item) => {
            return item === item.toLowerCase() ? item.toUpperCase() : item.toLowerCase()
        }).join('')


        return res
    }
    console.log(letter(str)); 
</script>